1. Field of the Invention
This invention relates to a method and means for extending the operating temperature range of spectroscopic window materials which are used to contain fluids at high pressures. The invention may be applied to any type of optical element exposed to such operating conditions.
2. Background
In general, the mechanical hardness and the mechanical strength of materials is reduced at elevated temperatures. In spectroscopy, where it are required to examine a fluid sample (for example, a liquid, gas or supercritical fluid) under high pressure and high temperature, the fluid must be contained in a vessel with at least one optically transmitting element which retains the fluid. Generally, this optical element becomes the limiting factor for both the pressure and temperature range of the vessel because of the lower mechanical strength of optical materials in comparison to metals. In many cases, the element is a disc-shaped window, though many other forms may be used, such as a lens, a multiple internal reflection element, an optic fibre, etc. In the case of a disc shaped-window, the dimensions of the disc (particularly the thickness and unsupported area) are chosen in relation to the pressure it is required to bear considering the mechanical strength of the optical material of the disc and allowing a substantial safety factor (often four-fold) in the calculations. Materials such as zinc sulphide (ZnS) and zinc selenide (ZnSe) are often preferred because of the spectral range over which they transmit in the infrared region and because their useful mechanical strength means they are suitable for high pressure applications. However, at elevated temperatures (say above 100.degree. C.) the pressure rating of such windows must be reduced as a consequence of their loss in mechanical strength. The behaviour of such windows may also be unpredictable under these high temperature and high pressure operating conditions; failure may be catastrophic, possibly with the window disintegrating forming dangerous splinters which may be ejected from the system with explosive force. This invention addresses these and other problems and shows how the operating pressure/temperature range may be extended with much improved safety. Pressures of the order of 7MPa to 30MPa are commonly of interest in industry. This invention is not restricted t this pressure range but does overcome the problem of providing satisfactory optical elements for use in chemical and related industries.